Silicon is subject to certain limitations, and industry is looking for a replacement. The electronics industry has net annual sales of over USD 200 billion, and this means that the development is being fuelled by powerful forces.
Scientist Johannes Svensson from the Department of Physics at the University of Gothenburg has investigated the manufacture and use of carbon nanotubes in his PhD thesis.
Faster and smaller
“I don’t believe that it will be cheaper to build transistors from another material than silicon, but carbon nanotubes can be used to produce smaller and faster components. This will also result in computers that consume less energy” says Johannes Svensson.
The amazing development in computer power that has taken place after the invention of the integrated circuit in the 1950s has been made possible by the transistor, which is the most important component of all processors, becoming ever-faster.
Increase the speed
The most common semiconductor material in transistors is silicon, since it is cheap and easy to process. But silicon has its limitations. As the size of the transistors is reduced in order to increase their speed, problems arise that lead to, among other things, increased energy consumption and large variation in the transistor properties.
By exchanging the silicon in the channel for a carbon nanotube, the transistors can be made both smaller and faster than today’s transistors. A carbon nanotube is a molecule in form of a hollow cylinder with a diameter of around a nanometer (roughly 1/50,000 of the width of a human hair) which consists of pure carbon. Some carbon nanotubes are semiconducting, and this means that they can be used in transistors, although there are several problems that must be solved before they can be connected together to form large circuits.
“Carbon nanotubes grow randomly and it is not possible to control either their position or direction. Therefore I have applied an electrical field to guide the tubes as they grow”, says Johannes Svensson.
Built his own
One of the effects of the electric field is that most of the carbon nanotubes lie in the same direction.
“In order to show that it is possible to build electronic components that contain only carbon nanotubes, I have built a transistor which not only has a carbon nanotube as its channel, but also another nanotube which is used as the electrode that controls the current.”
Another problem that must be solved when integrating nanotubes into larger circuits is the difficulty of manufacturing good metal contacts for the tubes. Johannes’ research has shown that the properties of the contacts depend on the diameter of the nanotubes. Choosing the correct diameter will allow good contacts with a low resistance to be achieved.
The thesis Carbon Nanotube Transistors: Nanotube Growth, Contact Properties and Novel Devices was successfully defended at a disputation held on 7 May 2010.